Concurrency: model-based design 1 - Computer Sciencekena/classes/5828/s09/lectures/23-model... ·...

Concurrency: model-based design 1 ©Magee/Kramer 2nd Edition


Chapter 8

Model-Based Design


Design

Concepts: design process: requirements to models to implementations

Models: check properties of interest: - safety on the appropriate (sub)system - progress on the overall system

Practice:model interpretation - to infer actual system behavior threads and monitors

Aim: rigorous design process.


♦  goals of the system ♦  scenarios (Use Case models) ♦  properties of interest

8.1 from requirements to models

Requirements

Model

♦  identify the main events, actions, and interactions ♦  identify and define the main processes ♦  identify and define the properties of interest ♦  structure the processes into an architecture

♦  check traces of interest ♦  check properties of interest

Any appropriate

design approach

can be used.


a Cruise Control System - requirements

When the car ignition is switched on and the on button is pressed, the current speed is recorded and the system is enabled: it maintains the speed of the car at the recorded setting.

Pressing the brake, accelerator or off button disables the system. Pressing resume or on re-enables the system. buttons


a Cruise Control System - hardware

Wheel revolution sensor generates interrupts to enable the car speed to be calculated.

Parallel Interface Adapter (PIA) is polled every 100msec. It records the actions of the sensors: •  buttons (on, off, resume)

•  brake (pressed)

•  accelerator (pressed)

•  engine (on, off).

buttons

engine

acceleratorbrake

PIApolled

wheel interrupt

CPU

throttleD/A

Output: The cruise control system controls the car speed by setting the throttle via the digital-to-analogue converter.


model - outline design

♦ outline processes and interactions.

Input Speed monitors the speed when the engine is on, and provides the current speed readings to speed control.

Sensor Scan monitors the buttons, brake, accelerator and engine events.

Cruise Controller triggers clear speed and record speed, and enables or disables the speed control.

Speed Control clears and records the speed, and sets the throttle accordingly when enabled.

Throttle sets the actual throttle.

Sensors

Prompts Engine

speed setThrottle


model -design

♦  Main events, actions and interactions. on, off, resume, brake, accelerator engine on, engine off, speed, setThrottle clearSpeed,recordSpeed, enableControl,disableControl

♦  Identify main processes. Sensor Scan, Input Speed, Cruise Controller, Speed Control and Throttle

♦  Identify main properties. safety - disabled when off, brake or accelerator pressed.

♦ Define and structure each process.

Sensors

Prompts


model - structure, actions and interactions

set Sensors = {engineOn,engineOff,on,off, resume,brake,accelerator} set Engine = {engineOn,engineOff} set Prompts = {clearSpeed,recordSpeed, enableControl,disableControl}

SENSOR

SCAN

CRUISE

CONTROLLER

Sensors

INPUT

SPEEDSPEED

CONTROL

setThrottle

speed

Engine Prompts

CONTROL CRUISE

CONTROL

SYSTEM

THROTTLE

The CONTROL system is structured as two processes. The main actions and interactions are as shown.


model elaboration - process definitions

SENSORSCAN = ({Sensors} -> SENSORSCAN). // monitor speed when engine on

INPUTSPEED = (engineOn -> CHECKSPEED), CHECKSPEED = (speed -> CHECKSPEED |engineOff -> INPUTSPEED ).

// zoom when throttle set THROTTLE =(setThrottle -> zoom -> THROTTLE).

// perform speed control when enabled SPEEDCONTROL = DISABLED, DISABLED =({speed,clearSpeed,recordSpeed}->DISABLED | enableControl -> ENABLED ), ENABLED = ( speed -> setThrottle -> ENABLED |{recordSpeed,enableControl} -> ENABLED | disableControl -> DISABLED ).


model elaboration - process definitions

set DisableActions = {off,brake,accelerator} // enable speed control when cruising, disable when a disable action occurs

CRUISECONTROLLER = INACTIVE, INACTIVE =(engineOn -> clearSpeed -> ACTIVE |DisableActions -> INACTIVE ), ACTIVE =(engineOff -> INACTIVE |on->recordSpeed->enableControl->CRUISING |DisableActions -> ACTIVE ), CRUISING =(engineOff -> INACTIVE |DisableActions->disableControl->STANDBY |on->recordSpeed->enableControl->CRUISING ), STANDBY =(engineOff -> INACTIVE |resume -> enableControl -> CRUISING |on->recordSpeed->enableControl->CRUISING |DisableActions -> STANDBY ).


model - CONTROL subsystem

||CONTROL =(CRUISECONTROLLER ||SPEEDCONTROL).

- Is control enabled after the engine is switched on and the on button is pressed? - Is control disabled when the brake is then pressed? - Is control re-enabled when resume is then pressed?

Animate to check particular traces:

•  Safety: Is the control disabled when off, brake or accelerator is pressed? •  Progress: Can every action eventually be selected?

However, we need analysis to check exhaustively :


model - Safety properties

Safety properties should be composed with the appropriate system or subsystem to which the property refers. In order that the property can check the actions in its alphabet, these actions must not be hidden in the system.

Safety checks are compositional. If there is no violation at a subsystem level, then there cannot be a violation when the subsystem is composed with other subsystems. This is because, if the ERROR state of a particular safety property is unreachable in the LTS of the subsystem, it remains unreachable in any subsequent parallel composition which includes the subsystem. Hence...



Is CRUISESAFETY violated?

||CONTROL =(CRUISECONTROLLER ||SPEEDCONTROL ||CRUISESAFETY ).

property CRUISESAFETY = ({DisableActions,disableControl} -> CRUISESAFETY |{on,resume} -> SAFETYCHECK ), SAFETYCHECK = ({on,resume} -> SAFETYCHECK |DisableActions -> SAFETYACTION |disableControl -> CRUISESAFETY ), SAFETYACTION =(disableControl->CRUISESAFETY). LTS?



Safety analysis using LTSA produces the following violation:

Trace to property violation in CRUISESAFETY: engineOn clearSpeed on recordSpeed enableControl engineOff off off

Strange circumstances! If the system is enabled by switching the engine on and pressing the on button, and then the engine is switched off, it appears that the control system is not disabled.



engineOn clearSpeed on recordSpeed enableControl engineOff engineOn speed setThrottle speed setThrottle …

The car will accelerate and zoom off when the engine is switched on again!

What if the engine is switched on again? We can investigate further using animation …

… using LTS? Action hiding and minimization can help to reduce the size of an LTS diagram and make it easier to interpret …


Model LTS for CONTROLMINIMIZED

engineOn

off

brake

accelerator

speed

off

brake

acceleratorengineOff

on

speed

off

brake

accelerator

engineOff

on

speed

engineOn

off

brake

accelerator

speed

speed off

brake

accelerator

engineOff

on

resume

speed

0 1 2 3 4 5

minimal ||CONTROLMINIMIZED = (CRUISECONTROLLER ||SPEEDCONTROL ) @ {Sensors,speed}.

… using progress?


model - Progress properties

Progress violation for actions: {accelerator, brake, clearSpeed, disableControl, enableControl, engineOff, engineOn, off, on, recordSpeed, resume} Trace to terminal set of states:

engineOn clearSpeed on recordSpeed enableControl engineOff engineOn

Cycle in terminal set: speed setThrottle

Actions in terminal set: {setThrottle, speed}

Check the model for progress properties with no safety property and no hidden actions


model - revised cruise controller

Modify CRUISECONTROLLER so that control is disabled when the engine is switched off: … CRUISING =(engineOff -> disableControl -> INACTIVE |DisableActions -> disableControl -> STANDBY |on->recordSpeed->enableControl->CRUISING ), …

Modify the safety property: property IMPROVEDSAFETY = {DisableActions,disableControl,engineOff} -> IMPROVEDSAFETY |{on,resume} -> SAFETYCHECK ), SAFETYCHECK = ({on,resume} -> SAFETYCHECK

|{DisableActions,engineOff} -> SAFETYACTION |disableControl -> IMPROVEDSAFETY ),

SAFETYACTION =(disableControl -> IMPROVEDSAFETY). OK now?


revised CONTROLMINIMIZED

engineOn

off

brake

accelerator

speed

off

brake

accelerator

engineOff

on

speed

off

brake

accelerator

engineOff

on

speed

off

brake

accelerator

engineOff

on

resume

speed

0 1 2 3

No deadlocks/errors


model analysis

||CONTROL = (CRUISECONTROLLER||SPEEDCONTROL||CRUISESAFETY )@ {Sensors,speed,setThrottle}.

||CRUISECONTROLSYSTEM = (CONTROL||SENSORSCAN||INPUTSPEED||THROTTLE).

We can now proceed to compose the whole system:

Deadlock? Safety?

No deadlocks/errors

Progress?


model - Progress properties

Progress checks should be conducted on the complete target system after satisfactory completion of the safety checks.

Progress checks are not compositional. Even if there is no violation at a subsystem level, there may still be a violation when the subsystem is composed with other subsystems. This is because an action in the subsystem may satisfy progress yet be unreachable when the subsystem is composed with other subsystems which constrain its behavior. Hence...

No progress violations detected. Progress?


model - system sensitivities

||SPEEDHIGH = CRUISECONTROLSYSTEM << {speed}.

Progress violation for actions: {engineOn, engineOff, on, off, brake, accelerator, resume, setThrottle, zoom} Path to terminal set of states:

engineOn tau

Actions in terminal set: {speed}

The system may be sensitive to the priority of the action speed.

What about progress under adverse conditions? Check for system sensitivities.


model interpretation

Models can be used to indicate system sensitivities.

If it is possible that erroneous situations detected in the model may occur in the implemented system, then the model should be revised to find a design which ensures that those violations are avoided. However, if it is considered that the real system will not exhibit this behavior, then no further model revisions are necessary.

Model interpretation and correspondence to the implementation are important in determining the relevance and adequacy of the model design and its analysis.


The central role of design architecture

Design architecture describes the gross organization and global structure of the system in terms of its constituent components.

We consider that the models for analysis and the implementation should be considered as elaborated views of this basic design structure.


8.2 from models to implementations

Model

Java

♦  identify the main active entities - to be implemented as threads

♦  identify the main (shared) passive entities - to be implemented as monitors

♦  identify the interactive display environment - to be implemented as associated classes

♦  structure the classes as a class diagram


cruise control system - class diagram

SpeedControl interacts with the car simulation via interface CarSpeed.

enableControl()disableControl()recordSpeed()clearSpeed()

Applet

CruiseControl

Controllerbrake()accelerator()engineOff()engineOn()on()off()resume()

SpeedControl

CarSimulator

CarSpeedsetThrottle()getSpeed()

Runnable

CruiseDisplay

car

control

sc

disp

disp

cs

CRUISECONTROLLER SPEEDCONTROL


cruise control system - class Controller class Controller { final static int INACTIVE = 0; // cruise controller states final static int ACTIVE = 1; final static int CRUISING = 2; final static int STANDBY = 3; private int controlState = INACTIVE; //initial state private SpeedControl sc; Controller(CarSpeed cs, CruiseDisplay disp) {sc=new SpeedControl(cs,disp);} synchronized void brake(){ if (controlState==CRUISING ) {sc.disableControl(); controlState=STANDBY; } } synchronized void accelerator(){ if (controlState==CRUISING ) {sc.disableControl(); controlState=STANDBY; } } synchronized void engineOff(){ if(controlState!=INACTIVE) { if (controlState==CRUISING) sc.disableControl(); controlState=INACTIVE; } }

Controller is a passive entity - it reacts to events. Hence we implement it as a monitor


cruise control system - class Controller

synchronized void engineOn(){ if(controlState==INACTIVE) {sc.clearSpeed(); controlState=ACTIVE;} } synchronized void on(){ if(controlState!=INACTIVE){ sc.recordSpeed(); sc.enableControl(); controlState=CRUISING; } } synchronized void off(){ if(controlState==CRUISING ) {sc.disableControl(); controlState=STANDBY;} } synchronized void resume(){ if(controlState==STANDBY) {sc.enableControl(); controlState=CRUISING;} } }

This is a direct translation from the model.


cruise control system - class SpeedControl class SpeedControl implements Runnable { final static int DISABLED = 0; //speed control states final static int ENABLED = 1; private int state = DISABLED; //initial state private int setSpeed = 0; //target speed private Thread speedController; private CarSpeed cs; //interface to control speed private CruiseDisplay disp; SpeedControl(CarSpeed cs, CruiseDisplay disp){ this.cs=cs; this.disp=disp; disp.disable(); disp.record(0); } synchronized void recordSpeed(){ setSpeed=cs.getSpeed(); disp.record(setSpeed); } synchronized void clearSpeed(){ if (state==DISABLED) {setSpeed=0;disp.record(setSpeed);} } synchronized void enableControl(){ if (state==DISABLED) { disp.enable(); speedController= new Thread(this); speedController.start(); state=ENABLED; } }

SpeedControl is an active entity - when enabled, a new thread is created which periodically obtains car speed and sets the throttle.


cruise control system - class SpeedControl

synchronized void disableControl(){ if (state==ENABLED) {disp.disable(); state=DISABLED;} }

public void run() { // the speed controller thread try { while (state==ENABLED) {

double error = (float)(setSpeed-cs.getSpeed())/6.0; double steady = (double)setSpeed/12.0; cs.setThrottle(steady+error);//simplified feed back control wait(500);

} } catch (InterruptedException e) {} speedController=null; }

SpeedControl is an example of a class that combines both synchronized access methods (to update local variables ) and a thread.


Summary

 Concepts   design process:

from requirements to models to implementations   design architecture

 Models   check properties of interest safety: compose safety properties at appropriate (sub)system progress: apply progress check on the final target system model

 Practice   model interpretation - to infer actual system behavior   threads and monitors

Aim: rigorous design process.


Course Outline

2.  Processes and Threads

3.  Concurrent Execution 4.  Shared Objects & Interference

5.  Monitors & Condition Synchronization

6.  Deadlock

7.  Safety and Liveness Properties

8.  Model-based Design

9.  Dynamic systems

10.  Message Passing

11.  Concurrent Software Architectures

Concepts Models Practice

12.  Timed Systems

13.  Program Verification

14.  Logical Properties

The main basic

Advanced topics …

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